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HOW many must die or come within a whisker of death before government officials admit what they don’t know?

The magic number may be four.

A 61-year-old woman lay in critical condition in Manhattan’s Lenox Hill Hospital yesterday, the latest casualty in the anthrax war that so far has claimed three lives.

It took this grave case of inhalation anthrax to prompt health officials to whisper a truth they’ve resisted even thinking until now:

“We don’t know what the hell we’re dealing with.”

Since the anthrax attacks began, there’s been a maddening disconnect between what officials told us, and what we can see with our own eyes.

While staff of the Centers for Disease Control and Prevention soothed us as if we were hysterical toddlers, Gov. Pataki and members of Congress publicly scrambled for Cipro.

It was late – but welcome – to hear the CDC’s Dr. Steven Ostroff admit befuddlement yesterday.

The case of the anthrax-stricken Kathy Nguyen is “troubling” Ostroff said, because “there’s no clear linkage with the mail.”

Nguyen worked in a supply room of a Manhattan hospital, not a media outlet or post office, although her office may have once been joined with the mail room.

Tom Ridge, the Bush administration’s director of homeland security, also admitted yesterday he was bewildered over the source of the woman’s infection.

In New Jersey, another baffling case of skin anthrax. That 51-year-old victim says she hasn’t been inside a post office for months.

“This is a mystery,” admitted George DeFerdinando, acting state health commissioner.

Here at The Post, where two people have skin anthrax – the first diagnosed Oct. 18 – and a third case is now suspected, “preliminary” test results on the office itself came back Monday, indicating that 14 “hot zones” exist.

To be fair, the Maryland lab enlisted to perform the tests were swamped with samples from post offices, said Dr. Debra Berg of the city Health Department.